A hydrogel contains water as the solvent and thus is useful as a gel having high biocompatibility. Such a hydrogel is used in various fields such as applications for commodities such as paper diapers, cosmetics and aromatics.
Examples of a conventional hydrogel include polymer gels formed through such steps that polymer chains are cross-linked to form a three-dimensional network structure, and that a noncovalent bond is formed between the three-dimensional network structure and the solvent such as water, so that the three-dimensional network structure swells to form a polymer gel. Many studies for the physical properties of the polymer gel and many developments of the applications of the polymer gel have been performed with respect to natural polymer gels formed from polysaccharide such as agarose and protein, and synthetic polymer gels in which polymer chains are cross-linked to each other through a chemical covalent bond, such as an acrylamide gel.
Recently, besides the above gels containing polymer compounds, hydrogels formed by the self-assembly of an organic compound having a relatively low molecular weight have been found and various hydrogels have been studied.
The formation of the gel by the self-assembly of a low molecular weight compound has been elucidated to occur through the following steps: in a substances (low molecular weight compounds) group in a random state at first, molecules of the substances group associate spontaneously while they are having directionality by, for example, an intermolecular non-covalent interaction between the molecules of the substances under an appropriate external condition (in a medium) to form a macro molecule-assembly: and plural macro molecule-assemblies form a network and the network swells with a surrounding solvent to form a gel. Examples of a driving force for this molecule association (self-assembly) include a force by an action of a hydrogen bond having a relatively weak bonding strength, and a force by a van der Waals interaction (non-hydrogen bond) having a bonding strength further weaker than that of the hydrogen bond.
Many of the low molecular weight gelators that have been disclosed are an amphipathic compound having a combination of a hydrophobic portion of a long-chain alkyl group and a hydrophilic portion. Examples thereof include an amphipathic compound in which the hydrophilic portion is an amino acid [Non-patent Document 1], an amphipathic compound in which the hydrophilic portion is a peptide [Patent Documents 1 and 2], an amphipathic compound in which the hydrophilic portion is a monosaccharide or a polysaccharide [Non-patent Documents 2 and 3], and an amphipathic compound in which the hydrophilic portion is a polyol [Non-patent Document 4]. In addition, there has also been disclosed a low molecular weight gelator utilizing such a tendency that a peptide made up with valine takes easily a β-sheet structure [Non-patent Document 5].
Such a low molecular weight hydrogelator can form a hydrogel by a method including: heating and stirring the hydrogelator and water as the solvent under a temperature condition of about 100° C. to dissolve and disperse the gelator in water; and leaving the resultant solution to stand still at room temperature.